See para 5.24...
Standard credit
5.21 A credit customer is a customer who pays on receipt, including customers that pay by debit or credit card, BACs transfer or cheque, and is neither a prepayment customer nor a Direct Debit customer.
5.22 For credit customers the supplier should provide the EBSS payment by one of the following methods: a. Crediting the account by an amount equal to the EBSS payment; or b. Reducing the amount (including Value Added Tax) charged to the account by an amount equal to the EBSS payment.
5.23 The credit should appear as it would if the customer had made a credit payment. Where the customer account is in credit or at zero balance, the EBSS payment amount will be added to the credit balance. Where a customer account is in debit the EBSS payment can be used against this.
5.24 The credit or reduction should be provided in the first five working days of each delivery month. The EBSS payment must be provided on a monthly basis, regardless of whether the account is paid monthly, quarterly or has an associated payment card.
Taken from here
I'm an Eon Next dual fuel customer with no particular expertise but have some time on my hands that I am using to try and help out a bit.